Fully depleted SOI devices are promising candidates in the future CMOS technology for the solution of the problem of the variability of the threshold voltage caused by random channel doping and short channel effects. These problems represent severe obstacles for further down-scaling. Alternative approaches comprise FinFET devices as well as Extremely Thin SOI (ETSOI) devices. The present invention relates to the improvement of the manufacture of the latter ones.
Currently, the vast majority of ETSOI wafers are manufactured using the Smart Cut® process. Those wafers are used to make chips with transistors operating in fully depleted mode. The uniformity of the thin silicon layers is crucial for the performance of the resulting ETSOI devices. In order to achieve a satisfactory uniformity of a thin silicon layer, an etch stop layer is deposited on a donor wafer and the thin silicon layer is grown on top of the etch stop layer. During the wafer transfer process the donor-wafer is bonded to a target wafer, for example, by a buried layer, particularly a buried oxide layer (BOX). Detachment is achieved either by splitting initiated in the silicon substrate of the donor wafer or splitting initiated in the etch stop layer. In the former case, remaining silicon above the etch stop layer is to be removed and subsequently the etch stop layer is also to be removed. When splitting is performed in the etch stop layer the step of removal of remaining silicon from the donor wafer is avoided.
If a boron-doped etch stop layer is provided, a sufficiently thick etch stop layer that allows for accurate control of the splitting initiated in the etch stop layer can readily be achieved. However, direct contact of the thin silicon layer to a boron-doped etch stop layer results in some diffusion of the boron dopants into the thin silicon layer thereby deteriorating the quality of the ETSOI configuration. On the other hand, if an SiGe layer is used as the etch stop layer, relaxation defects pose a severe problem for a thickness of the SiGe layer that would allow for splitting initiated in the SiGe etch stop layer. This is due to the fact that a relatively high Ge concentration is needed to provide for a sufficiently high etching selectivity. Thus, in this case splitting has to be performed in the bulk substrate of the donor wafer and the above-mentioned step of removal of silicon material remaining on the surface of the etch stop layer after the detachment cannot be avoided. What is more, etching of the remaining silicon necessarily results is some under-etching of the thin silicon layer at edges of and under the etch stop layer transferred to the target substrate.
In view of the above, it is an object of the present invention to provide a method for the manufacture of thin SOI devices wherein the etching problem is alleviated.